I understand that evolution is constant process that acts on a population in successive generations. Thus, it is obvious that evolution is happening. However, I'm curious as to the stricking examples of evolution that have occurred since Darwin's publication of the theory.

Essentially, how different are we now from the humans at the time at which evolution was conceived?

Yes, the epigenetic, or Lamarckian, response is important +1. The environment - air/water quality, physical activity, availability of energy-dense food, antibiotics and other drugs - have all exerted influences on the epigenome.
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Larry_ParnellOct 3 '12 at 18:26

@Alan Boyd - Epigenetic changes have been tracked across multiple generations, so they are inheritable. Obese mice will sire offspring who weigh more despite given the same diet and exercise as a control mouse. The effect can last for several generations, but that's a good question. I want to say yes, since evolution really just relies on inheritability and selection - but I'm not sure if epigenetic modifications could become permanent.
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MCMOct 4 '12 at 5:11

Well, "On the origin of species" was published in 1859. If we assume that for humans a generation is approximately 20 years, that means that there have been between seven and eight generations since 1859. That is nowehere near enough time to observe evolutionary changes.

So, in answer to your question, not very different at all.

EDIT:

I think I need to clarify my answer. When we talk about a species evolving or a species being different today than it was X years ago, we are referring to the species as a whole. Of course evolution is an ongoing process, selective pressures are shaping our gene pool constantly. However, in order to say that humans are different today to those of 1859, we would need to show that the prevalence of particular genotypes has changed. That a specific trait (a gene or a mutation in a gene) has become "fixed", or, at the very least, that it has spread to a significant percentage of the population.

@niallhaslam's answer gives a link to a wired.com article about genetic resistance to HIV. This is not something that has recently evolved in response to HIV but is the result of an ancestral mutation. The article itself states that:

Using formulas that estimate how long genetic mutations have been
around, researchers have discovered that the mutation dates to the
Middle Ages.

Granted, this could be a good example of selection in action. If, after many, many more generations, this mutation has spread to the majority, or a significant percentage, of the population, we could point to it as an example of how humans have changed under the selective pressure of HIV infection. In order for this to happen, HIV will have to become common enough that having a resistance to is beneficial to a very large subset of humanity. I would expect to see such selective pressure in, for example, countries like South Africa where 12% of the population is HIV positive. Such pressure, however, will be negligible in countries with lower infection rates. In the US, for example, a recent study found HIV incidence among persons aged 13 years and older to be 42,200–54,000. Since the US has ~312 million people, 54,000 represents ~0.02% of the population. This will not provide enough of a selective pressure to observe differences in the short term evolutionary scale.

We do not, in fact, "evolve quickly in response to parasites viruses etc". Pathogens may apply selective pressure yes. But this is not a quick process. In any case, we do not evolve in response to them. Mutations occur, randomly usually, and if they confer an advantage they may be selected for. Said advantage may be because they confer resistance to disease but even then, the original mutation was not "in response" to the pathogen, However, such selection happens over many successive generations and will spread relatively slowly (quickly by evolutionary standards perhaps, but still slow for us, and certainly slower than 7 generations) across the population. The same holds true of epigenetic changes. Even if they can (as now seems likely) be transmitted down the generations, any selective advantage they confer will still act at the generational level.

So, while we are, of course, constantly evolving, there are no significant differences between the humans of today and those of Darwin's times. Sure, some genotypes may exist today that did not then and vice versa. These, however, are not representative of the species' genome. They may become so after many generations but, even if they exist, they have not had time to propagate through the gene pool and cannot be taken as evidence that H. sapiens as a species is "different" today.

In my opinion you are taking a very narrow view of what evolution means. However, with your clarification I have removed my downvote.
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Noah SnyderOct 4 '12 at 13:21

I am not expressing an opinion on evolution as such. Simply on what it would take to be able to say that a species has changed (has "evolved" as much as I hate that term in this context) since date X.
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terdonOct 4 '12 at 13:26

I would say that a species has changed as soon as the frequency of some gene has changed. Thus all species are always changing, and no generation is exactly like the previous one.
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Noah SnyderOct 4 '12 at 13:29

Even that is hard to define. For a species to have "changed" that change needs to be persistent. Otherwise it may just be a random fluke in a specific generation that will not survive to the next. Unless the frequency of a given mutation remains different over a few successive generations I do not consider that the "species" has changed.
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terdonOct 4 '12 at 13:33

They are indeed recent. The authors claim that the most recent changes were ≈6,600 years ago. While very recent indeed from an evolutionary point of view, they are still slightly older than Darwin :).
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terdonOct 4 '12 at 12:15

@terdon: Unless I'm missing something, isn't that 6,600 an estimated average age? They are discussing several hundred different genes, some may be more recent. Furthermore, as they say, the timeframe is not precise and may only indicate when the gene became common.
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Noah SnyderOct 4 '12 at 13:18

I am just pointing out that the time frame here is not of the same order of magnitude. They show no evidence for changes that have become fixed in the last couple of hundred years. I only skimmed the article though, so please let me know if I am wrong.
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terdonOct 4 '12 at 13:26

I agree that they don't appear to produce specific evidence of that any of their examples are in the 200 year time frame. However, I don't think there's anything in the paper that precludes some of them being very recent.
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Noah SnyderOct 4 '12 at 13:33

Nor do they offer any evidence that they are very recent so I see no reason to assume that it is true. At least in my experience (in my defense, I did my PhD on comparative genomics), evolutionary changes simply are not observable in the kind of time frame (~7 generations) discussed here.
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terdonOct 4 '12 at 13:38